I remember my mom regularly having the store bought version and a cup of coffee for breakfast when I was a kid. My husband now loves the home made version. I'm pretty sure I'll never eat one from a box again.
Preheat oven to 350 degrees. Lightly spray cookie sheet or line with parchment paper.
Combine flour, salt, baking soda, cinnamon and oats and set aside.
Beat butter and sugars until creamy, then mix in molasses, vanilla and eggs.
Gradually add dry ingredients to creamed mixture. I do this in 3 additions, mixing well after each. Place cookie dough in fridge while mixing filling.
In a small dish, dissolve the salt in the hot water and then allow to cool to room temp. Mix the shortening and marshmallow cream well, then add the powdered sugar and vanilla. Mix on high til fluffy. Scrape the sides of your bowl often. Add the salt water and mix well. This is where you may have to add in some extra powdered sugar. You want it fluffy and not sticky. Set the filling aside. You do not have to chill.
If you don't have a cookie scoop, drop in tablespoon size scoops on prepared cookie sheet. Bake 10 - 12 minutes. Cookies should be golden brown. Be careful no to over bake so that your cookies will stay moist.
Spread filling on flat side of cooled cookie and top with second cookie and enjoy!
Note: For storing, if you're stacking these cookies in a container, put wax paper between layers so they won't stick together. Or, wrap each pie in plastic wrap. I wrapped mine so that my husband can take to work for a snack, plus I've also put some in the freezer.